The present invention relates to a method of etching a thin film, such as a polysilicon film.
Generally, film forming processes for forming films on a semiconductor wafer or a glass substrate and etching process for pattering the films are carried out alternately to form a desired number of devices when fabricating a semiconductor integrated circuit or a LCD panel.
When forming, for example, a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an etching process for etching a polysilicon film in forming the gate electrode of the MOSFET is very important because the length of the gate electrode of the MOSFET is an important factor dominating the electric characteristic of the MOSFET. Accordingly, the polysilicon film must be etched over the entire surface of the wafer to form the gate electrode in an accurate length, which requires highly accurate, uniform processing.
In most cases, an inductive coupled plasma processing system capable of operating at a low process pressure on the order of several mTorrs is employed for carrying out a polysilicon film etching process because the process pressure of a conventional diode parallel-plate plasma etching system is excessively high and the diode parallel-plate plasma etching system is unable to etch the surface of the polysilicon film in a sufficiently high uniformity.
FIG. 9 is a typical view of a generally known inductive coupled plasma processing system. The inductive coupled plasma processing system has a substantially cylindrical processing vessel 2 defining a reaction chamber. A wafer table 4 is placed in the processing vessel 2 to support a semiconductor wafer W thereon. The interior space of the processing vessel 2 is evacuated through an annular exhaust opening formed in the bottom of the processing vessel 2 so as to surround the wafer table 4.
A plasma producing vessel 8 of a diameter smaller than that of the top wall of the processing vessel 2 is joined to the top wall of the processing vessel 2 so as to open into the reaction chamber. The plasma vessel 8 defines a plasma producing chamber. An induction coil 12 is mounted on the plasma producing vessel 8 and is connected to, for example, a 13.56 MHz radio frequency power source 10.
An etching gas, such as chlorine gas (Cl2), supplied through a gas inlet part 14 formed on the top wall of the plasma producing vessel 8 is ionized by an electromagnetic field created by an induction coil 12 to produce a plasma. Chlorine molecules or chlorine atoms are activated by the plasma to produce radicals. Chlorine gas is supplied into the plasma producing chamber at a flow rate of about 125 sccm when the volume of the plasma producing chamber is 59 litters. The radicals flow from the plasma producing chamber into the reaction chamber of the processing vessel 2. The radicals are guided and dispersed by a tapered guide plate 16 so as to fall evenly on the surface of the wafer W to etch, for example, a polysilicon film formed over the surface of the wafer W.
The behavior or flow of the gas or radicals flowing from the plasma producing vessel 8 into the processing vessel 2 in an evacuated atmosphere of a pressure on the order of several mTorrs is very complicated. It has been a common knowledge that radicals spread by diffusion in an atmosphere of such a low pressure and fall substantially uniformly on the surface of the wafer W.
However, when a film formed on a wafer is etched by the foregoing etching method, the difference between an etch rate in a peripheral part of the wafer and an etch rate in a central part of the wafer is considerably large and, in some cases, the film formed on the surface of the wafer cannot uniformly be etched. Irregularity in etch rate in the surface of a wafer is not a significant problem with 6 in. and 8 in. wafers, but is a significant problem with 12 in. (30 cm) wafers, and a film formed on such a large-diameter wafer cannot be etched in a desired uniformity.
The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to provide an etching method capable of etching a film formed on a surface of a wafer at greatly improved uniform etch rate.
The inventors of the present invention simulated flows of etching gases and radicals, in which gas supply rate was varied in a wide range, and found that the flux by diffusion on a peripheral part of a wafer is not changed significantly but the flux by flow is changed greatly by flow, and the uniformity of etch rate over the surface of the wafer can be improved when the etch gas is supplied at a high etching gas supply rate. The present invention has been made on the basis of such a knowledge acquired through simulation.
According to one aspect of the present invention, an etching method comprises an etching gas supply step of supplying an etching gas through a gas supply port into a plasma producing chamber, a plasma producing step of producing radicals in the plasma producing chamber by converting the etching gas into a plasma by applying radio frequency power to the etching gas, and an etching step of etching an object to be processed in a reaction chamber, which is connected to the plasma producing chamber and is evacuated, by the radicals flowing from the plasma producing chamber into the reaction chamber, in which the etching gas is supplied through the gas supply port at an etching gas supply rate of 8.4 sccm or above for a substantial volume of one liter of the reaction chamber.
The supply of the etching gas at 8.4 sccm or above for one liter of substantial volume of the reaction chamber improves the uniformity of etching over the surface of the object to be processed as well as etch rate.
In this etching method, the plasma producing step may convert the etching gas into a plasma by inductive coupling using an induction coil and the etching step may use, for example, chlorine gas as the etching gas and may etch a polysilicon film formed on the object to be processed by using chlorine gas.